The disclosed subject matter relates generally to integrated circuit manufacturing and, more particularly, to a method and apparatus for optimizing models for extracting dose and focus from critical dimension data.
The formation of various integrated circuit (IC) structures on a wafer often relies on lithographic processes, sometimes referred to as photolithography, or simply lithography. As is well known, lithographic processes can be used to transfer a pattern of a photomask (also referred to herein as a mask or a reticle) to a wafer.
For instance, patterns can be formed from a photoresist layer disposed on the wafer by passing light energy through a mask having an arrangement to image the desired pattern onto the photoresist layer. As a result, the pattern is transferred to the photoresist layer. In areas where the photoresist is sufficiently exposed, and after a development cycle, the photoresist material becomes soluble such that it can be removed to selectively expose an underlying layer (e.g., a semiconductor layer, a metal or metal containing layer, a dielectric layer, a hard mask layer, etc.). Portions of the photoresist layer not exposed to a threshold amount of light energy will not be removed and serve to protect the underlying layer during further processing of the wafer (e.g., etching exposed portions of the underlying layer, implanting ions into the wafer, etc.). Thereafter, the remaining portions of the photoresist layer can be removed.
There is a pervasive trend in the art of IC fabrication to increase the density with which various structures are arranged. For example, feature size, line width, and the separation between features and lines are becoming increasingly smaller. In these sub-micron processes, yield is affected by factors such as mask pattern fidelity, optical proximity effects and photoresist processing. Some of the more prevalent concerns include line end pullback, corner rounding and line-width variations. These concerns are largely dependent on local pattern density and topology.
As semiconductor device technology migrates to the 45-nm node and beyond, process capability for critical lithography steps are becoming increasingly difficult to maintain. The resolution of the photolithography process determines the smallest feature size that can be repeatedly produced on a wafer:
      R    =                            k          1                ⁢        λ            NA        ,
where λ is the wavelength and NA is the numerical aperture.
Depth of focus (DOF) is the distance a wafer can depart from best focus before the image quality is lost.
  DOF  =                              k          2                ⁢        λ                    2        ⁢                              (            NA            )                    2                      .  
Hence, by increasing the NA and reducing λ, resolution is improved, but depth of focus is reduced.
At previous technology nodes, the depth-of-focus (DOF) for critical patterning layers was large enough to assume that focus variation would not have a significant impact on output quality metrics. As this assumption becomes less realistic, new methodologies are needed to control focus. On the equipment side, traditional techniques for periodic calibration of machine focus are proving incapable of addressing subtle variations that occur on products at small geometries. On product, run-to-run APC control of critical dimensions (CD) using exposure dose alone relies on the underlying assumption that focus deviations will not significantly impact the approximately linear relationship between dose and CD.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.